In conventional vehicles, fuel consumption is reduced by reducing the rolling resistance of tires to suppress heat build-up. A demand for lower fuel consumption of vehicles owing to tires is becoming increasingly greater, and thus further studies are essential for improving fuel economy. Natural rubber generally used in tires has been considered to have better fuel economy than styrene-butadiene rubber; however, styrene-butadiene rubber is now being improved to give lower fuel consumption. Therefore, natural rubber is also required to give much lower fuel consumption in order to achieve good fuel economy of the tire as a whole.
In order to improve natural rubber in terms of fuel economy, modification of natural rubber has been proposed. For example, Patent Document 1 discloses a modification method in which a natural rubber latex is mixed with a surfactant and then washed. In this method, however, proteins and a gel fraction are removed to some extent but still in an insufficient level; further reduction in tan δ is desired. In addition, tires are also required to have properties such as heat-aging resistance; however, no documents teach simultaneous achievement of good fuel economy and good heat-aging resistance.    Patent Document 1: JP 3294901 B